AP Biology · Lesson 2 of 8 · Unit 7

Population genetics: mutation, genetic drift and gene flow

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Population genetics (Topic 7.4) explains how non-selective, random processes alter allele frequencies in a gene pool. On the AP Biology exam, you will analyze allele frequency data across generations and scenarios—such as habitat fragmentation or island colonization—to distinguish genetic drift and gene flow from natural selection.

By the end you’ll be able to

Explain how mutation, a random process, adds new genetic variation to a population.
Explain genetic drift, including the bottleneck and founder effects, and why it can make a small population diverge from other populations of the same species.
Explain how gene flow from migration adds or removes alleles and prevents two populations from diverging into separate species.
Describe how changes in allele frequencies over time provide evidence for evolution, and state a null hypothesis for a population study.

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Core ideas (continued)
Worked examples
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What the exam asks

Section I (Multiple Choice): Stimulus sets often show graphs of allele frequencies fluctuating across multiple small populations or tables comparing mainland and island gene pools. You must identify which evolutionary mechanism is operating and predict its effect on genetic diversity.
Section II (Free Response): Frequently appears in Q4 (Conceptual Analysis), where a disruption (like a wildfire or a new highway) alters a population, or Q2/Q3, where you must state a null hypothesis regarding whether an environmental factor or barrier changes allele frequencies.

Core ideas

Evolution as a change in the gene pool

A population’s gene pool consists of all copies of every allele at every locus in all individuals of that population. Microevolution is defined as a change in allele frequencies in a population over generations. Anytime data show that the frequency of allele $p$ or $q$ changes significantly over time, evolution has occurred.

Mutation: the ultimate source of new alleles

Mutations are random changes in the nucleotide sequence of DNA.

Mutation is the only process that creates brand-new alleles (new genetic variation).
Mutations are random: they occur through DNA replication errors or mutagen exposure, never because an organism “needs” a trait to survive.
By itself, mutation changes allele frequencies very slowly, as mutation rates at any single locus are low. However, it provides the raw variation upon which genetic drift and natural selection act.

Genetic drift: random sampling in small populations

Genetic drift is a non-selective process in which allele frequencies change unpredictably from one generation to the next purely by chance (random sampling error during reproduction and mortality).

Drift has the strongest effect in small populations, where a few random deaths or matings noticeably shift the gene pool.
Over time, drift reduces genetic variation within a population through the random loss of alleles ( $p$ or $q = 0$ ) and the fixation of one allele ( $p$ or $q = 1.0$ ).
Because drift is random in each isolated group, it causes different small populations of the same species to genetically from one another.